Our long-term objective is to understand the neural control of individuated movements, those in which one body part moves relatively independently of the motion or posture of other body parts. Individuated movements are the first lost and last recovered when neurologic lesions affect the motor cortex or corticospinal tract, and play an increasingly important role in the motor repertoire of primates, especially humans. Modern evidence contradicts the notion that movements of different fingers are controlled from distinct, somatotopically arrayed regions of the primary motor cortex (M1), is if labeled lines extended from separate cortical regions to each finger. We hypothesize instead that each finger movement is controlled by a network of neurons distributed throughout the M1 hand region, and that this network reorganizes during motor skill learning. To test these hypotheses experimentally, we will identify M1 neurons that provide direct connections to spinal motoneuron averaging of electromyographic (EMG) activity. Each CM neuron and its target muscles will be recorded simultaneously during performance of 12 individuated finger and wrist movements. By comparing the activity of CM neurons and their target muscles across these 12 movements, we will (Aim 1) examine where CM neurons lie along a spectrum of functional possibilities-from labeled-lines to diversified elements of a distributed network; and (Aim 2) the extent to which the activity of CM neurons combined through the physiologically identified connections to spinal motoneuron pools, can account for the patterns of EMG activity actually recorded. We also will use the identified CM neuron-target muscle connections to map the spatial distribution of output effects from M1 to muscles at the single neuron level to determine (Aim 3) whether outputs to selected muscles that act on radial versus ulnar digits are spatially segregated or entirely overlapping. Finally, we will map M1 repeatedly during motor skill learning to determine (Aim 4) whether any spatial segregation diminishes as the M1 territories that provide output to trained muscles progressively enlarge and increasingly overlap.